As the death toll in the coronavirus pandemic approaches the grim milestone of 6 million, scientists are still trying to determine why some people have more serious infection outcomes than others. Aside from age, health, and other individual factors, one early variable has been the presence of air pollution, specifically particulate matter with a diameter of 2.5 micrometers or less (PM2.5). In April 2020, Harvard University researchers found that small increases in particulate matter can produce large effects in the death rate. A recent study from Italy has taken a closer look, examining what happened to residents of one city from the start of the pandemic through March 2021.
Low-Level Exposure Risk
There has been growing evidence that air pollution may heighten the risk of severe illness from COVID-19. For instance, in 2020, researchers found that increased exposure to such hazardous air pollutants as formaldehyde is associated with a 9% increase in deaths among people with COVID-19. Another 2020 study by scientists in Europe and the United States estimated that particulate air pollution contributed about 15% to worldwide COVID-19 mortality.
Published in Occupational and Environmental Medicine, the recent Italian prospective study looked at Varese, a municipality near the Swiss border that is the eighth-largest city in the Lombardy region. Northern Italy has pockets of air where pollution exceeds limits set by the European Union, so researchers wanted to see how those levels affected residents over a roughly 1-year period.
The researchers were motivated to carry out the study because most of the previous investigations had fewer granular data—information at the county, province, or state level. They were also limited to the first half of 2020.
“In our analyses, we were able to take into account most of the demographic and clinical characteristics that may increase SARS-CoV-2 susceptibility other than long-term exposure to airborne pollutants,” said corresponding author Giovanni Veronesi, a professor at the Research Center in Epidemiology and Preventive Medicine, Department of Medicine and Surgery, at the University of Insubria in Italy. “In addition, we offer a number of sensitivity analyses, some based on the careful peer review process, to strengthen the confidence on our results. We believe we have offered the first solid empirical evidence for the association between long-term exposure to air pollution and the incidence of COVID-19.”
Veronesi and his colleagues collaborated with universities, regional health institutions, and Arianet, an environmental consultancy, to track 62,848 people aged 18 or older in Varese. On the basis of their addresses and air quality measurements, these residents were linked to exposure to pollutants, including PM2.5. There were 4,408 cases of coronavirus infection during the period and a yearly average of PM2.5 of 12.5 micrograms per cubic meter. The study found an extra 294 cases per 100,000 people a year and concluded that “an increase in exposure to long-term airborne pollutants of 1 [microgram per cubic meter] is associated with a 2–5% increase in the rate of COVID-19 incidence.” Sensitivity analyses confirmed the association, according to the study.
Veronesi said he was surprised to find that there was an increased COVID-19 risk even with relatively low concentrations of PM2.5.
“This really aligns well with the idea that continuous exposure to even low levels of air pollution makes people more susceptible to diseases—and it reinforces the need of deep [social] modifications in order to have healthier lives,” said Veronesi.
Possible Endemic Threat
Heather Walton, a senior lecturer in environmental health at Imperial College London who was not involved in the study, said that although the statistical analysis needs to be confirmed, the findings add to existing knowledge.
“There are only a few studies that have individual data available. There have already been other studies on long-term exposure to air pollution and hospital admissions that had individual data, but, at least until May 2021…there was not one like this on incidence of infection. However, this is subject to whether the statistical analysis is appropriate,” said Walton, who coauthored an Imperial College review on pollutants and COVID-19. “The hypotheses addressed are familiar ones. Nonetheless, if the statistics check out, then it is good to have additional evidence on the hypotheses.”
Veronesi noted that more research is required to establish a causal link between air pollution and COVID-19 susceptibility. He and his collaborators are working to expand the study to cover more people, a longer period, nonurban settings, and broader COVID-19 end points, such as hospitalizations and deaths.
“If SARS-CoV-2 is to become endemic in a population, our results imply that this will be an additional health threat to people already suffering higher respiratory and cardiovascular disease rates due to air pollution exposure,” Veronesi said. “Therefore, governments should implement their efforts to reduce air pollution levels as a measure to mitigate the public health burden of COVID-19, with no further delays.”
—Tim Hornyak (@robotopia), Science Writer